1. Field of the Invention
The invention relates to an optical component comprising a substrate, on which at least one curved substrate surface is formed that defines an optical axis of the optical component, wherein the substrate surface is coated with a multilayer coating that is active in the ultraviolet light region at a design wavelength λ0 and comprises a first layer, applied to the substrate surface, made from a first dielectric material and at least one second layer, applied to the first layer, made from a second dielectric material. The invention also relates to an optical imaging system comprising at least one such component, and to a method for producing optical components.
2. Description of the Related Art
In many areas of the application of optical systems, there is a rising need for powerful optical components whose optical properties are optimized for design wavelengths λ0 in the deep ultraviolet (DUV) region or in the vacuum ultraviolet (VUV) region, in particular in the wavelength region between approximately 120 nm and approximately 260 nm. Radiation from this wavelength region is used, for example, in microlithography systems for manufacturing large-scale integrated semiconductor components or other finely structured components with the aid of wafer steppers or wafer scanners. In this case, a light source, for example a laser, illuminates via an illumination system a mask (reticle) whose image is imaged with the aid of a projection system onto a semiconductor wafer coated with a photoresist layer.
Since the cost-effectiveness of this method is decisively determined by the speed of an exposure operation, there is a need for optical systems with the lowest possible light losses between light source and wafer. Consequently, those surfaces of transparent optical components of the systems that are exposed to radiation are coated with so-called antireflection layers (AR layers) or reflection-reducing layers in order to increase their transparency or transmission. These antireflection coatings lead to an increase in transmission as long as the light losses that are introduced, for example, by absorption and scattering by the coating, remain small by comparison with the order of magnitude of the reduction in reflection. The reduction in reflection also serves to avoid false light or scattered light that can impair the imaging properties of high quality imaging systems.
In order to be able to produce ever finer structures, it is attempted, on the one hand, to enlarge ever further the image-side numerical aperture (NA) of the projection objective and of the illumination systems adapted thereto. On the other hand, ever shorter wavelengths are being used, for example 248 nm, 193 nm and 157 nm. Since only a few sufficiently transparent materials whose Abbé constants are, moreover, relatively close to one another are available in this wavelength region for the purpose of manufacturing the optical components, it is difficult to provide purely refractive system with adequate correction of chromatic aberrations. Consequently, widespread use is made for this wavelength region of catadioptric systems in which refracting and reflecting components, in particular lenses and mirrors with a curved reflective surface, are combined. The curved substrate surface of a concave mirror can be coated with a dielectric multilayer coating that serves to optimize the reflectance, and is denoted as a reflective coating or highly reflecting coating (HR layer). The use of concave rear surface mirrors (Mangin mirror) in catadioptric systems is likewise known.
Optical materials that are sufficiently transparent even at wavelengths below 193 nm principally include fluoride crystal materials such as monocrystalline calcium fluoride (CaF2) or barium fluoride (BaF2).
Again, a selection of available dielectric materials for constructing dielectric multilayer coatings with alternating high refractive index and low refractive index individual layers is limited in the case of known short wavelengths. At wavelengths of 193 nm or below, fluoride materials are preferred on the basis of their high transparency or a low material-specific absorption. Use is frequently made in this case of lanthanum fluoride (LaF3) as high refractive index dielectric material, and of magnesium fluoride (MgF2) as low refractive index dielectric material.
Studies on the growth behavior of lanthanum fluoride layers on calcium fluoride substrates and substrates made from synthetic silica glass are presented in the article entitled “Hetero-epitaxial growth and optical properties of LaF3 and CaF2” by Y. Taki and K. Muramatsu in: Thin Solid Films 420-421 (2002) pages 30 to 37. The authors demonstrate that lanthanum fluoride layers that have been applied to calcium fluoride substrates at temperatures of 250° C. by vacuum deposition and whose surfaces run parallel to (111) net planes (crystal lattice plane) of the crystalline substrate material grow epitaxially in an orderly fashion. When the lanthanum fluoride was deposited under the same vapor deposition conditions onto a substrate made from synthetic silica glass, the lanthanum fluoride layer had a polycrystalline structure with statistical crystallite orientation. The authors indicate that a defective crystallinity and a porous structure of lanthanum fluoride layers impair the optical transparency of the latter in the vacuum ultraviolet (VUV) region, while the epitaxial growth of lanthanum fluoride on <111>-oriented calcium fluoride very effectively diminishes the photoabsorption in the lanthanum fluoride layer.
WO 03/009015 presents optical components with calcium fluoride substrates whose surfaces are parallel or at an angle of at most 30° to a (111) plane of the crystal material. These substrate surfaces are coated with an epitaxially grown layer made from lanthanum fluoride and which is intended to have a dense structure with few defects, a high refractive index and low absorption losses.
The patent U.S. Pat. No. 5,963,365 presents various three-layer antireflection coatings that are intended to have a strongly reflection-reducing effect for design wavelengths in the range between 150 nm and 300 nm. It is proposed in this case to make the first layer, next to the substrate, from a low refractive index material, for example MgF2 or Na3AlF6, with thin optical layer thicknesses of between 0.05 λ0 and 0.15 λ0, while the high refractive index and low refractive index layers lying thereupon are designed as quarter wavelength layers (optical layer thickness approximately 0.25 λ0), LaF3 being used respectively as high refractive index material. These multilayer coatings are intended to have a good reflection-diminishing effect on flat or weakly curved substrates made from synthetic silica glass up to a large incidence angle of the incident radiation of more than 40°.
The patent U.S. Pat. No. 6,261,696 B1 describes a coating method, the aim of which is to facilitate in conjunction with the use of fluoride-containing substrate materials the avoidance of the formation of color centers when coatings are produced with the aid of sputter techniques. A layer made from silicon oxide, beryllium oxide, magnesium oxide or magnesium fluoride with a layer thickness of 30 nm or less is applied as first layer to the substrate surface for this purpose. The protective layer is intended to prevent the plasma from penetrating into the substrate material, and to prevent color centers from being produced thereby.
Particularly for applications in the field of microlithography at high numerical apertures, optical components made from fluoride crystal material can occur that have strongly curved surfaces. It has been shown in investigations by the inventor that with such systems undesired variations in the transmission whose causes were unexplained sometimes occurred over the cross section of the optical system.